Fabrication of butt-type strain isolation seals



R. ROTH Nov. 20, 1962 FABRICATION OF' BUTT-TYPEv STRAIN ISOLATION SEALS Filed May 9, 1958 2 Sheets-Sheet 1 Nov. Z0, 1962 R. ROTH 3,064,331

FABRICATI N F' BUTT-TYPE STRAIN ISOLATION SEALS Filed May 9, 1958 INVENTOR. RQBEHT Run-1 BY MM W44 ,4f/guy Robert Roth, Lancaster, Pa., assigner to Radio Corporation of Ameri-ca, a corporation of Delaware Filed May 9, 1953, Ser. No. 734,181 3 Claims. (Cl. 29-1555) This invention relates to ceramic-to-metal seals and particularly to a method of assembling a butt-type strain isolation ceramic-to-metal seal mount in the fabrication of large power electron tubes.

The terms seal and its derivatives, as used herein, are intended to describe a condition wherein metal and/ or inslative parts are rigidly joined and permanently bonded together in vacuum tight relation for use in an electron tube.

In certain large power electron tubes, massive or rigid, cylindrical, conductive, metal, electrode support members having flat surfaces are insulatingly mounted with respect to each other with separating ceramic rings. In such tubes, the vacuum envelope thereof may in part be built up of these conductive metal support members and ceramic ring assemblies. According to known practices, each metal member may be sealed to each ceramic member by first providing the ceramic member with an adherent metalized coating and then brazing the metal member thereto.

Since the massive metal support members are likely to have a coeflicient of expansion quite different from that of the ceramic member, such seals are sometimes caused to crack and develop leaks due to heating of the members during normal tube processing. To prevent seal cracking, strain isolation devices are used. One type of strain isolation device comprises a pair of thin-walled metal cylinders or rings disposed coaxially end-to-end with a ceramic ring sealed therebetween to the iiat end surfaces thereof. Such a structure may be referred to as a butt-type strain isolation insulator mount. According to prior art fabrication methods, the thin-walled cylinders are either integral with the massive conductive support members or first brazed to at end surfaces thereof before the ceramic ring is sealed in place.

Although seal cracking is considerably lessened by such a strain isolation device, difficulty is encountered in fabricating such a device free from dimensional and vacuumtight imperfections. Since the ceramic ring must be raised to a temperature of about 1500 C. in order to provide the necessary metalized coating thereon, it frequently becomes warped upon cooling. Although such warpage may be relatively minute, it nevertheless is often suicient to prevent a continuous or 360 contact of the ceramic ring with the flat end surfaces of the thin-Walled cylinders and thus result in vacuum leaks.

Moreover, it will be appreciated that according to prior art practices, determination of 360 vacuum tightness is possible only after the ceramic ring and thin-walled cylinders have been brazed into integrality with the massive support members of the electron tube. Thus, when 360 vacuum tightness is not achieved, considerable and costly portions of the electron tube must be scrapped.

According to the prior art, one way of insuring 360 contact is to use excessive amounts of solder to completely iill any valleys in the ceramic due to warpage. However, even if vacuum tightness is thus achieved, there still exists the problem of misalignment. If the two thinwalled cylinders are canted relative to each other in being sealed to a warped ceramic ring, the massive conductive support members may be forced into a corresponding misalignment. This then necessitates corrective machining of the surfaces of the support members. Such machining not only presents an economy consideration but also results in other quite serious alignment and mounting problems of the electrodes themselves. Moreover, excessive solder appears to cause an undesirable allo-ying which in itself may result in loss of strain isolation.

It is, therefore, an object of my invention to provide an improved method of assembling a butt-type strain isolation ceramic-to-metal seal mount in which warpage of the ceramic member does not deleteriously eect the quality of the seal, and in which undesired alloying due to excessive solder and corrective machining of the massive support members are avoided.

Briey, according to my invention, the thin-walled metal cylinders or rings of `a butt-type strain isolation mount are provided as separate members apart from the massive conductive support members and are rst sealed by a conventional brazing method to the ceramic ring. The thin-walled cylinders are forced into 360 or continuous contact with the ceramic ring by applying independently acting forces at a plurality of points equally spaced around the periphery thereof. The cylinders are thereby caused to conform to the Warped contour of the ceramic ring. After brazing the cylinders to the ceramic ring, their remote ends are trimmed to ilatness and parallelism and the three piece butt-type mount assembly sized to a predetermined overa-ll axial length. The mount assembly may then be leak checked before being integrated into an electron tube by brazing the liat trimmed end surfaces of the cylinders to the at surfaces of said massive support members.

ln the drawings:

FIG. 1 is an elevation view in cross section of a representative type of large power electron tube incorporating a butt-type strain isolation mount fabricated according to my invention;

FIG. 2 is an enlarged section view of the butt-type strain isolation mount of FIG. 1 illustrating the fabrication thereof according to my invention; and

FIG. 3 is a perspective view of preferred jigging apparatus usable in the practice of my invention.

1FlG. 1 illustrates a large power electron tube 10 incorporating a butt-type strain isolation ceramic-to-metal mount fabricated according to my invention. The tube i0 includes a massive conductive control grid support member 14, massive conductive cathode support member 18 and a massive conductive screen grid support member 26, each having dat end surfaces. The screen grid support member 26 includes a plurality of vanes 30 extending radially inward toward an anode 34. Foraminous screen grid structures 38 are mounted on the ends of the vanes 30. A plurality of iilamentary cathode strands 42 are mounted, one between each of two adjacent vanes 30 and are supported at one end by the cathode support member 18. Self-supporting tubular control grid members 46 are mounted one around each of said cathode strands 42 and are supported at one end on an annular grid foot member 50 attached to the control grid support member 14. A more detailed description of the specific electrode structures of the tube 10 and of the electron optics involved therein is presented in U.S. Patent 2,817,031 issued to W. P. Bennett on December 17, 1957.

In the tube 10, a butt-type strain isolation ceramic-tometal sea-l mount 56 serves to insulatingly mount the control grid support member 14 relative to the cathode support member 18. The butt-type mount 56 includes a pair of thin-walled metal cylinders 60 and 64, having initially flat end surfaces disposed in end-to-end relation with a ceramic ring 68 vacuum sealed therebetween according to known seal-making techniques.

grid members 46 with the other tube electrodes.

vachieve optimum tube performance.

.port member 18. As will be further appreciated, any

axial misalignment of these two support members will be reflected in the mounting alignment of the tubular control Such a rnisalignment is obviously intolerable.

According to 'prior art fabrication techniques, where axial misalignment of the two thin-walled cylinders results, alignment of the electrode arrays is restored by Vcorrective machining of certain of the surfaces of the control grid support member 1'4 or the cathode support member 18.v VFor example, .the surface 80 of the cathode support member 18 might be given a tapered cut of up to thousandths of an inch. Such corrective machining VAis not only expensive but upsets other dimensions which must be held to extremely close tolerances in order to It will be readily appreciated that by removing such an amount from the surface 80 of the 'cathode support member 18, differences in the tensioning of the individual cathode strands must necessarily result. This, in addition to the problem of kalignment ofthe electrode arrays as well as other .alignment problems in mounting, is Vavoided according to my invention.

FIG. 2 illustrates a preferred jigging method of fabricating the butt-type mount 56 according to my invention. The two thin-walled metal cylinders 60 and 64 andthe .previously metalized ceramic ring 68 are coaxially stacked as illustrated with a solder washer 84 interposed between each of the cylinders and the ceramic ring. A plurality Yof jigging ngers 88 (FIG. 3) are hooked over each of the cylinders 60 and 64 at equally spaced points around the periphery thereof and the jigged assembly axially compressed by a weight-plate'92. The jigging fingers simultaneously serve two purposes. They center the cylinders ,'60 and 64 relative to the ceramic ring 68, and they provide independently acting forces at points spaced around thecylinders for axially compressing the cylinders into 360 contact with the ceramic ring 68. The structure and action of the jigging fingers 88 will be more fully Adescribed hereinafter.

'The jiggedfa'ss'embly is then placed in a brazing furnace and heated to aY temperature sucient to melt the solder i rings`84 and bond the cylinders 60 and 64 to the ceramic ring 68. VAfter removal from the furnace, the mount 56 can be"shock and leak Vtested to insure perfect 360 vacuum tightness. By so doing, the possibility of having fto scrap expensive tube parts to which the mount 56 is to' be brazed is completely avoided. More important though, experience has shown that in fabricating butttype 'mounts according to my invention essentially no scrappage at allresults Vfrom failure to obtain 360 vacuum tightness. "With vacuum tightness thus assured, the

remotefends 100 of the cylinders 60 and 64 are then trimmed by machining or grinding to provide flatness and y' parallelism and va desired overall axial length of the mount 56. Theinished mount'56 can then be integrated into the electron tube 10 by brazing the metal cylinders 60"aud 64 toand between the dat surfaces of the control grid support member 14 and the cathode support member l18. AItrwill be vappreciated that since parallelism of the remote ends 100 of the mount 56 has been provided, little 'or no corrective'machining of'the control grid or cathode support members 14 and 18 will be required. Thus, it

-is readily apparent that advantages are obtained by a practice ofrmy invention which are totally impossible accordingto prior art techniques.

FIG. 3 shows the jigging linger 88 utilized in the assembly of FIG. 2 to comprise a thin metal block formed with an arm 96 having at its end a hook-like projection 9S. The arm 96 is so dimensioned that the inside length l is slightly less than half the difference of the ceramic ring OD and the thin-walled cylinder 1D. Thus, when hooked over one of the cylinders 6) or 64 and permitted to rest against the outer surface of the ceramic ring 68, the jigging finger 88 is canted. Axial compression then by the weight-plate 92 causes the jigging finger 88 to exert two forces on the cylinder 60 or 64. A rst force is exerted axially against the cylinders 60 and 64V pressing them against the warped ceramic ring 68 and into conformity therewith. A second force is exerted radially outward against the cylinders 60 and 64 and serve to center the cylinders relative to the ceramic ring 68.

In actual practice, warpage of the ceramic ring 68 may be only a few thousandths of an inch. Such warpage, although small, will result in the upper tips 104 of the jigging fingers 88 not being in a single plane as defined by the contacting surface 108 of the weight-plate 92. However, due to thesmallness of warpage and since the jigging ngers 88 also have a settling action due to radial displacement of the cylinders 60 and 64, the weight-plate 92 will contact all jigging lingers hooked over any one cylinder and thus serve to produce independently acting forces axially compressing that cylinder into 360 contact with the ceramic ring 68.

Although the jigging lingers 88 constitute preferred apparatus for practicing my invention, it will be appreciated that such are not essential thereto. Other means or apparatus can be substituted for the jigging fingers 88 Vprovided such means or apparatus'will produce the necessary separately acting forces around the periphery of the cylinders 60 and 64 to press them into 360 contact with the warped surfaces of the ceramic ring 68.

According to actual specifications used in fabricating the butt-type mount 56 as incorporated in one tube 10, the ceramic ring 68 consists of a high alumina-type ceramic. The ceramic ring 68 is provided with an O.D. of 3% inches, an LD. of 35/16 inches, and an axial thickness of 1,6 inch. The thin-walled metal cylinders 60 and 64 are identical and consist of OFHC copper. The cylinders are provided with an axial length of 0.235 inch, Ya wall thickness of 0.040 inch, and a mean diameter of 3.530

inches.

The ceramic ring 68 is metallized with a conventional Ymolybdenum powder sintered thereon at approximately 1500o in a reducing atmosphere. The metalized coating is subsequently provided with a -nickel coating on top thereof to promote wetting by the solder during the subsequent brazing of the thin-walled cylinders 60 and 64 Y thereto.V The solder washers 84 are of commercial BT silver solder and are .003 inch thick and have an O.D. of 3.700 inches and an I D. of 3.375 inches.

in jigging the cylinders 60' and 64, the ceramic ring'68,

Yand the solder washers 84 together as illustrated in FIG.

, minutes.

Prior to integrating into the electron tube Y1-0 the mount 56, provided according to the above specifications, the

mount is subjected both to shock test and vacuum-leak Y test. After undergoing a series of i000 G shocks, such of rigid envelope members having dat surfaces and an annular butt-type strain isolation insulator mount interv posed between said members, comprising the steps of first sealing the end surfaces of a rigid ceramic insulator ring to and between the adjacent end surfaces of a pairof axially-spaced eXi le rings, while axially compressing said flexible rings into continuous contact with said insulator ring, to form said insulator mount, then shaping the remote end surfaces of said liexible rings to the contour of said ilat surfaces of said envelope members, and then sealing the remote end surfaces of said rings to said fiat surfaces of said envelope members to complete said envelope portion.

2. -in the manufacture of an electron tube, tbe fabrication of a vacuum-tight envelope portion comprising a pair of rigid metal envelope members having iiat surfaces and an annular butt-type strain isolation insulator mount interposed between said members, comprising the steps of first brazing the end surfaces of a rigid ceramic ring to and between the adjacent end suraces of a pair of axiallyspaced thin-Walled metal cylinders, While axially compressing said metal cylinders into continuons contact with said ceramic ring, to form said insulator mount, then shaping the remote end surfaces of said metal cylinders to the contour of said at surfaces of said envelope members, and then 'orar/:ing said shaped end surfaces of said metal cylinders to said at surfaces of said metal envelope members to complete said envelope portion.

3. in the manufacture of an electron tube, the fabrication of a vacuum-tight envelope lportion comprising a pair of rigid metal envelope members having ilat surfaces and an annular butt-type strain isolation insulator mount interposed between said members, comprising the steps of iirst brazing the end surfaces 0i a rigid ceramic ring to and between the adjacent end surfaces oi a pair of axiallyspaced coaxial thin-walled metal cylinders, while axially compressing said metal cylinders into continuous contact with said ceramic ring by applying a plurality of inde pendently acting substantially axiaily parallel compressive forces to said cylinders and said ring, to form said insulamount, tiren machining the remote end surfaces of said metai cylinders to the contour of said fiat surfaces of said envelope members, and then brazing said machined end surfaces of said metal cylinders to said flat surfaces of said metal envelope members to compiete said envelope portion.

References Cited in the Jdie of this patent UNITED STATES PATENS 1,562,533 Weintraub Nov. 24, 1925 2,431,226 Bei-key et al Nov. 18, 1947 2,707,757 Agule May 3, 1955 2,800,710 Dunn july 3G, 1957 2,845,698 Giovannucci et al Aug. 5, 1958 2,882,116 Williams Apr. 14, 1959 

